Grapefruit contains a chemical that interferes with an enzyme that controls how drugs are absorbed through the intestines, resulting in a potentially toxic dose of medication.

Other citrus fruits that contain the chemical to some degree include limes, pomelos and Seville oranges, which are often used in marmalade.

"Many of the drugs that interact with grapefruit are highly prescribed and are essential for the treatment of important or common medical conditions," says Bailey, lead author of the study in Monday's issue of the Canadian Medical Association Journal.